The aim of this work has been to identify factors which determine the likelihood that an 8 to 13 month-old baby will avoid or be distressed by the approach of an unfamiliar adult. Infant's reactions to strangers and the determinants of these reactions are much more complex than has been acknowledged in the literature. Our work indicates that few experiential or temperamental factors are related to reaction to strangers, but relatively low exposure to active, but not familiar adults in the second quarter of life may predispose infants to react negatively to strangers. We have also shown that disagreements in the literature are due in part to differences in scoring which can lead to quite different conclusions about how babies react to strangers.